Photoinitiators for use in intaglio printing inks

ABSTRACT

An intaglio printing ink, curing by free radical acrylate chemistry, which does not fluoresce in at least the visible region under ultraviolet light is prepared by using an acylphosphine oxide as the photoinitiator.

The present invention relates to the use of certain acylphosphine oxidesas photoinitiators in energy-curable (e.g. ultraviolet-curable) intaglioprinting inks.

Security documents are preferably printed by the intaglio printingprocess. The term “intaglio printing” as used in this application refersto the so-called “engraved steel die” or “copper plate” printingprocesses which are well known to those skilled in the art. The printingplates used herein are usually chromium plated, engraved nickel platesor cylinders, made by galvanic replication of an—oftenhand-engraved—original copper plate. The following does not apply to thealso well known rotogravure or gravure printing processes, which rely ona different type of ink.

In intaglio printing, ink is applied under pressure to the engravedsurface of a cylinder. Thus, not only does the ink fill the engravingsof the cylinder, it is also applied to the planar non-image surface ofthe cylinder. The ink is then thoroughly wiped from the planar surfaceof the engraved cylinder before the printing process is carried out.This is commonly effected by a wiping cylinder contrarotating to theengraved cylinder so that the two surfaces which touch are moving inopposite directions. Given the right conditions and, crucially, theright ink, this will remove the surplus ink from the planar surface aswell as a small amount of ink from the surface of the ink in theengravings, so that the only ink on the engraved cylinder is in theengravings. The substrate to be printed is then passed between theengraved cylinder and an impression material, which is typically anothercylinder, with the application of considerable pressure between theengraved cylinder and the impression material, which is a hard butdeformable material. The considerable pressure deforms the impressionmaterial, forcing the substrate to be printed into the engravings on theengraved cylinder. This results in the substrate picking up some ink,corresponding to the engravings on the surface of the engraved cylinder.The ink then has to be dried.

Because of the unique characteristics of intaglio printing, the inksused for other forms of printing, for example lithographic printing,cannot be used for intaglio, and the formulations tend to be completelydifferent.

Plainly, the rheology of the ink is critical to its success. For oilbased intaglio inks, the addition of organic solvent permits adjustmentof rheology, and at the same time facilitates wiping of the excess inkfrom the non-image areas of the plate. Such solvents are not used inenergy curable inks which are therefore tackier, and require the use ofplasticiser to achieve the same effect.

In addition to the rheology and ease of removal of surplus ink,discussed above, intaglio printing inks must meet the followingrequirements:

-   -   They must remain on the engraved cylinder until the moment of        printing when they must transfer readily and in a consistent        manner to the substrate to be printed.    -   They must have good film-forming properties and the cured inks        must be sufficiently flexible that they remain intact even when        the printed matter (e.g. banknotes) is subject to abuse.    -   Once the substrate has been printed, the ink must not transfer        back to other surfaces with which it may come into contact,        especially other printed matter.    -   The cured ink must have excellent chemical and mechanical        resistance so as to withstand the many diverse materials and        conditions to which banknotes may be subject.    -   They must be safe for handling by all members of the public,        including the very young.

It is also self-evident that, where the ink is to be cured by energy,e.g. ultraviolet or electron beam, any components added to the ink toachieve any of the above requirements must not interfere with the cure.Not surprisingly, it is difficult to meet all of these desideratasimultaneously.

The majority of intaglio inks in current use are printed on sheetfedpresses. The main drying mechanism is by oxidation of the resin and oilcomponent in the stack of printed matter, a process that typically takesseveral days to become substantially complete. This drying process hasthe disadvantage that wet ink can transfer to the unprinted side whilstthe stack is being built, or, indeed, may transfer as a consequence ofany movement of the stack during the period in which oxidative drying istaking place. Such a transfer of ink to the unprinted side is known inthe industry as “set-off”, and is normally considered a fault.

As a result, energy-curable, and specifically, ultraviolet (UV)-curableintaglio printing inks have been proposed. The use of a vehicle systemthat can be cured under the action of UV light offers immediate dryingand eliminates the occurrence of set-off. For example, GB 1466470discloses a UV-curable ink for copperplate intaglio printing whichcomprises specific amounts of a curable binder which is an ester oramide of acrylic acid, a pigment, a photoinitiator, an activator for thephotoinitiator and an inert extender permeable to ultraviolet light.EP1,260,563 discloses UV intaglio ink formulations which arewater-washable and which can easily be precipitated from the wipingsolution at the post-wiping stage.

However, the main practical use nowadays of intaglio printing issecurity printing, for example of banknotes, and this commonly requiresthat the ink used should not fluoresce in the visible region under UVlight. Specifically, banknotes are commonly printed on non-fluorescingpaper and so a first test to determine if a note is counterfeit is toplace it under a UV source. If the note fluoresces blue, it is almostcertainly counterfeit. Against this background the printer has theoption of applying inks that themselves either do or do not fluoresce,or indeed a combination of both types. Thus, in order to make best useof the security possibilities, it is important that the ink maker hassufficient flexibility of formulation to produce both fluorescent andnon-fluorescent inks. Since the photoinitiator combinations proposed foruse in the above patents do fluoresce in the visible region under UVlight, this precludes the production of non-fluorescing inks, and theformulations cannot, therefore, in practice be used for a large numberof the applications for which they would otherwise be expected to beuseful.

We have now surprisingly found that acylphosphine oxide photoinitiators,which are free radical photoinitiators, do not fluoresce in the visibleregion under UV light and so can be used in energy-curable intaglioprinting inks for security applications.

Acylphosphine oxides are known for use as photoinitiators in variousother types of printing ink. For example, U.S. Pat. No. 6,777,459 B2describes the use of compositions with mono and bis acyl phosphineoxides, and describes their applicability to UV curing inks, includingscreen print, flexographic, gravure and off-set printing inks. U.S. Pat.No. 4,710,523 and U.S. Pat. No. 4,298,738 describe some acylphosphineoxides and their use as photoinitiators in photopolymerisable surfacecoatings, finishes and printing inks. However, they do not suggest thatthese compounds may be used in intaglio printing inks, nor that theresulting inks do not fluoresce in the visible region under UV light.

Thus, the present invention consists in an energy curable intaglioprinting ink, curing by free radical acrylate chemistry, and including aphotoinitiator comprising an acylphosphine oxide, whereby the ink doesnot fluoresce in at least the visible light wavelength region whenexposed to ultraviolet light.

“The visible light region” of the spectrum is that region normallyvisible to humans, and is generally in the range from 400 to 700 nm.

A preferred class of compounds for use in the present invention arethose compounds of formula (I):

in which:R¹ and R² are independently selected from C₁-C₁₂ alkyl groups, C₃-C₇cycloalkyl groups, aryl groups, aralkyl groups, heterocyclic groupshaving from 3 to 7 ring atoms, of which at least one is an oxygen,sulphur or nitrogen atom and groups of formula —COR³;or R² represents a group of formula —OR⁴, where R⁴ represents a C₁-C₆alkyl group, an aryl group, an aralkyl group or a cationic group oratom, or R² represents a group of formula (II):

where X represents a C₁-C₁₈ alkylene group or a biphenyldiyl group, andR⁵ represents any of the groups represented by R¹ or a group of formula—OR⁴; andR³ represents a C₁-C₆ alkyl group, an aryl group, a heterocyclic grouphaving from 3 to 7 ring atoms, of which at least one is an oxygen,sulphur or nitrogen atom, or a group of formula (IV):

where Y represents a C₁-C₁₈ alkylene group a phenylene group, acyclohexylene group or a biphenyldiyl group.

In the compounds of formula (I), where R¹ and/or R² represents an alkylgroup, this may be a straight or branched chain alkyl group having from1 to 12 carbon atoms. Examples of such groups include the methyl, ethyl,propyl, isopropyl, butyl, isobutyl, sec-butyl, t-butyl, pentyl,isopentyl, neopentyl, hexyl, isohexyl, heptyl, octyl, nonyl, decyl,undecyl and dodecyl groups. Of these groups, we particularly preferthose having from 1 to 8 carbon atoms, more preferably from 1 to 6carbon atoms. The most preferred groups are those having from 1 to 4carbon atoms. Where R³ and/or R⁴ represents an alkyl group, this may bea straight or branched chain group having from 1 to 6, preferably from 1to 4, carbon atoms and examples of such groups include those listedabove having that number of carbon atoms.

Where R¹ and/or R² represents a cycloalkyl group, this has from 3 to 7ring carbon atoms, and examples include the cyclopropyl, cyclobutyl,cyclopentyl, cyclohexyl and cycloheptyl groups, of which the cyclopentyland cyclohexyl groups are preferred.

Where R¹ and/or R² and/or R³ and/or R⁴ represents an aryl group, this isa group having, preferably, from 6 to 14 carbon atoms in an aromaticcarbocyclic ring, and examples include the phenyl, naphthyl, anthryl andphenanthryl groups, of which the phenyl group is preferred. Such groupsmay be substituted or unsubstituted. If substituted, there is noparticular restriction on the number of substituents, other than thoseimposed by the number of substitutable carbon atoms, and possibly bysteric constraints, however, in general, from 1 to 4, more preferablyfrom 1 to 3, substituents would be common. Examples of suitablesubstituents include the halogen atoms (e.g. chlorine, fluorine, bromineor iodine atoms), C₁-C₆ alkyl groups (e.g. those alkyl groups havingfrom 1 to 6 carbon atoms included in the examples of alkyl groupsrepresented by R¹ and/or R²), C₁-C₆ alkoxy groups, C₁-C₆ alkylthiogroups, and aryl groups (e.g. as exemplified here, and which may besubstituted or unsubstituted, as defined here, provided that any arylsubstituent may not itself be further substituted by an aryl group).

Where the substituent(s) on an aryl group is an alkoxy group, this maybe a straight or branched chain group having from 1 to 6 carbon atoms,of which examples include the methoxy, ethoxy, propoxy, isopropoxy,butoxy, isobutoxy, sec-butoxy, t-butoxy, pentyloxy, isopentyloxy,neopentyloxy and hexyloxy groups.

Where the substituent(s) on an aryl group is an alkylthio group, thismay be a straight or branched chain group having from 1 to 6 carbonatoms, of which examples include the methylthio, ethylthio, propylthio,isopropylthio, butylthio, isobutylthio, sec-butylthio, t-butylthio,pentylthio, isopentylthio, neopentylthio and hexylthio groups.

Where R¹ and/or R² and/or R⁴ represents an aralkyl group, this may be analkyl group, preferably having from 1 to 4 carbon atoms, which issubstituted by from 1 to 3 aryl groups, which may be as defined andexemplified above. Preferred examples of such aralkyl groups include thebenzyl, benzhydryl, trityl, phenethyl, 1-phenylethyl, 3-phenylpropyl,4-phenylbutyl and naphthylmethyl groups, of which the benzyl group ispreferred.

Where R¹ and/or R² and/or R³ represents a heterocyclic group, this hasfrom 3 to 7 ring atoms, of which at least one, and preferably 1 or 2,more preferably 1, is an oxygen atom, a nitrogen atom or a sulphur atom.More preferably, the group has 5 or 6 ring atoms, preferably of whichone is an oxygen, nitrogen or sulphur atom. Examples of such groupsinclude the thienyl, furyl, pyranyl, pyrrolyl, imidazolyl, pyrazolyl,pyridyl, pyrazinyl, pyrimidinyl, pyrrolidinyl, pyrrolinyl,imidazolidinyl, imidazolinyl, piperidyl, piperazinyl and morpholinylgroups, of which the thienyl and pyridyl groups are preferred.

Where R⁴ represents a cationic group or atom, this may be, for example,an ammonium group, or a metal atom (M)⁺ _(x), where M is a metal, e.g.sodium, potassium or lithium, and x is the reciprocal of the valence ofthe metal M.

Where X or Y represents an alkylene group, this may be a straight orbranched chain group having from 1 to 18 carbon atoms, and examplesinclude the methylene, ethylene, trimethylene, propylene,tetramethylene, pentamethylene, hexamethylene, heptamethylene,octamethylene, nonamethylene, decamethylene, undecamethylene,dodecamethylene, tridecamethylene, tetradecamethylene,pentadecamethylene, hexadecamethylene, heptadecamethylene, andoctadecamethylene groups.

A particularly preferred class of acylphosphine oxides to be used in thepresent invention are those compounds of formula (V):

in which:R¹ represents a C₁-C₁₂ alkyl group, a cyclohexyl group or an aryl group;andR³ is as defined above.

We particularly prefer compounds of formula (V) in which each R³ isindependently selected from phenyl groups and phenyl groups having from1 to 4 halogen and/or C₁-C₆ alkyl and/or C₁-C₆ alkoxy substituents.

Also preferred are compounds of formula (V) in which R¹ represents aC₁-C₁₂ alkyl group or a phenyl group which is unsubstituted or has from1 to 3 C₁-C₆ alkyl or alkoxy substituents.

Another particularly preferred class of acylphosphine oxides to be usedin the present invention are those compounds of formula (VI):

in which:R¹ and R³ are as defined above; andR^(2a) represents a C₁-C₁₂ alkyl group, a C₃-C₇ cycloalkyl group, anaryl group, an aralkyl group, a heterocyclic group having from 3 to 7ring atoms, of which at least one is an oxygen, sulphur or nitrogenatom, or a group of formula —OR⁴, where R⁴ is defined above.

The most preferred class of compounds of the present invention are thosecompounds of formula (VII):

in which:n is 0 or 1;R⁶ represents a C₁-C₁₂ alkyl group, a C₁-C₆ alkoxy group, a phenyl groupor a phenyl group having from 1 to 4 substituents selected from C₁-C₆alkyl groups, C₁-C₆ alkoxy groups and halogen atoms; andR⁷, R⁸, R⁹, R¹⁰, R¹¹ and R¹² are the same as or different from eachother and each represents a hydrogen atom, a C₁-C₆ alkyl group, a C₁-C₆alkoxy group or a halogen atom.

Specific examples of photoinitiators which may be used in the presentinvention include: methyl 2,6-dimethylbenzoyl-phenylphosphinate, methyl2,6-dimethoxybenzoyl-phenylphosphinate,2,6-dimethylbenzoyldiphenylphosphine oxide,2,6-dimethoxybenzoyldiphenylphosphine oxide, methyl2,4,6-trimethylbenzoyl-phenylphosphinate, ethyl2,4,6-trimethylbenzoyl-phenylphosphinate, potassium(2,4,6-trimethylbenzoyl)-(2′-hydroxybiphenyl-2-yl)phosphinate,2,4,6-trimethylbenzoyldiphenylphosphine oxide,2,4,6-trimethoxybenzoyldiphenylphosphine oxide,2,3,6-trimethylbenzoyldiphenylphosphine oxide, methyl2,4,6-trimethylbenzoyl-tolylphosphinate, ethyl2,6-dichlorobenzoyl-phenylphosphinate,2,6-dichlorobenzoyldiphenylphosphine oxide,2,6-dibromobenzoyldiphenylphosphine oxide,2-chloro-6-methylthiobenzoyldiphenylphosphine oxide,2,6-dimethylthiobenzoyldiphenyl-phosphine oxide,2,3,4,6-tetramethylbenzoyldiphenylphosphine oxide,2-phenyl-6-methylbenzoyldiphenylphosphine oxide, methyl2,4,6-trimethylbenzoyl-naphthylphosphinate, ethyl2,4,6-trimethylbenzoyl-naphthylphosphinate, ethyl2,6-dichlorobenzoyl-naphthylphosphinate,1,3-dimethylnaphthalene-2-carbonyl-diphenylphosphine oxide,1,3-dimethoxynaphthalene-2-carbonyl-diphenylphosphine oxide,1,3-dichloronaphthalene-2-carbonyl-diphenylphosphine oxide,2,8-dimethylnaphthalene-2-carbonyl-diphenylphosphine oxide,2,4,6-trimethylpyridine-3-carbonyldiphenylphosphine oxide,2,4-dimethylfuran-3-carbonyldiphenylphosphine oxide,2,4-dimethoxyfuran-3-carbonyldiphenylphosphine oxide, methyl2,4,5-trimethylthiophene-3-carbonyldiphenylphosphinate,2,4,5-trimethylthiophene-3-carbonyldiphenylphosphine oxide,bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentyl-phosphine oxide andbis(2,4,6-trimethylbenzoyl)phenylphosphine oxide.

Other acylphosphine oxide photoinitiators may also be used, for example1,10-bis[bis(2,4,6-trimethylbenzoyl)phosphine oxide]decane and acopolymer of 4-acryloyloxy-2,6-dimethylbenzoyl-diphenylphosphine oxideand butyl acrylate.

The most preferred acylphosphine oxides are 2,4,6-trimethylbenzoyldiphenylphosphine oxide, bis(2,4,6-trimethylbenzoyl)phenyl phosphineoxide, ethyl 2,4,6-trimethylbenzoyl diphenylphosphinate andbis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide.

These acylphosphine oxides are all known in themselves and may beprepared by known methods or by analogous methods to those known. Theyare described, for example in U.S. Pat. Nos. 4,298,738, 4,710,523 and6,777,459, the disclosures of which are incorporated herein byreference.

Although it is a feature of the present invention that thephotoinitiator comprises an acylphosphine oxide, if desired, otherphotoinitiators can be used in addition, provided that they also do notresult in a cured ink which fluoresces in the visible region under UVlight. For example, it would be possible to use a hybrid freeradical/cationic curing system, with the free radical photoinitiatorbeing the acylphosphine oxides of the present invention, and a suitablecationic initiator.

The printing inks of the present invention are designed to be cured byUV or EB (electron beam) radiation and typically include a bindercomprising one or more oligomers and/or reactive monomers. Formulationsare well-known and can be found in standard textbooks such as the series“Chemistry & Technology of UV & EB Formulation for Coatings, Inks &Paints”, published in 7 volumes in 1997-1998 by John Wiley & Sons inassociation with SITA Technology Limited.

Suitable oligomers (also referred to as prepolymers) include epoxyacrylates, acrylated oils, urethane acrylates, polyester acrylates,silicone acrylates, acrylated amines, acrylic saturated resins andacrylic acrylates. Further details and examples are given in “Chemistry& Technology of UV & EB Formulation for Coatings, Inks & Paints”, VolumeII: Prepolymers & Reactive Diluents, edited by G Webster, published in1997 by John Wiley & Sons in association with SITA Technology Limited.

Because of the high viscosity of most oligomers, diluents are requiredto reduce the overall viscosity of energy curing ink or coatingformulation, so as to assist in handling and application. Suitablediluents may include water or “reactive” monomers which are incorporatedinto the cured film. Reactive monomers are typically acrylates ormethacrylates, and can be monofunctional or multifunctional. Examples ofmultifunctional monomers would include polyester acrylates ormethacrylates, polyol acrylates or methacrylates, and polyetheracrylates or methacrylates. Further details and examples are given inthe book edited by G Webster (op. cit.).

To make inks suitable for the water wipe intaglio presses, the ink mustbe soluble in dilute caustic solutions. This can be achieved by usingacid functional resins. These may be acrylate or methacrylatefunctional, and therefore reactive, or inert in UV and EB systems.Suitable examples include styrene maleic anhydride resins, such asSMA1440F available from Cray Valley, and aromatic acid methacrylate andacrylate half esters.

The inks will contain pigments as the colouring agent. The pigment maybe any desired inorganic and/or organic pigment suitable for intaglioprinting such as CI Pigment Yellow 12, CI Pigment Yellow 42, CI PigmentYellow 93, CI Pigment Yellow 110, CI Pigment Yellow 173, CI PigmentBlack 7, CI Pigment Black 11, CI Pigment Orange 34, CI Pigment Red 9, CIPigment Red 22, CI Pigment Red 23, CI Pigment Red 57:1, CI Pigment Red67, CI Pigment Red 122, CI Pigment Red 146, CI Pigment Red 185, CIPigment Red 224, CI Pigment Red 242, CI Pigment Red 254, CI PigmentGreen 7, CI Pigment Green 36, CI Pigment Blue 15, CI Pigment Blue 15:3,CI Pigment Violet 23, CI Pigment Violet 32, or CI Pigment Violet 37.

Preferably, the ink will contain one or more fillers (also calledextenders) in an amount of about 1-35% based on the weight of thefinished ink. Suitable fillers include china clay, calcium carbonate,calcium sulphate, talc, silica, corn starch, titanium dioxide, aluminaand mixtures thereof.

The ink may also contain about 1 to 5%, based on the weight of thefinished ink, of a wax to improve scuff resistance. Suitable waxesinclude carnauba waxes, montan waxes, polytetrafluoroethylene waxes,polyethylene waxes, Fischer-Tropsch waxes, silicone fluids and mixturesthereof.

Other additives may be incorporated in the ink, including adhesivereagents, antifoaming reagents, levelling reagents, flow reagents,antioxidants, ultraviolet absorbers, flame retardants, etc.

In addition, if desired, a plasticiser may be incorporated into theprinting ink in order to facilitate the wiping process to remove surplusink from the engraved cylinder. Examples of suitable plasticisersinclude:

Citrates, for example: acetyl tri(2-ethylhexyl) citrate, acetyl tributylcitrate, acetyl triethyl citrate, tributyl citrate, tricyclohexylcitrate, triethyl citrate, and triisoamyl citrate;

Epoxidised oils, fatty acids and esters thereof, for example:2-ethylhexyl esters of epoxidised tall oil, epoxidised linseed oil,epoxidised soya fatty acid ethylhexyl ester, epoxidised soybean oil;

Fatty acids, which may be saturated or unsaturated, especially thosehaving a molecular weight within the preferred range given above, forexample hexanoic, octanoic, decanoic, lauric acid, myristic acid,palmitic acid, stearic acid, arachidic acid, behenic acid, lignocericacid, lauroleic acid (dodecenoic acid), pentadecanoic acid, margaricacid, myristoleic acid, palmitoleic acid, oleic acid, gadoleic acid(eicosenoic acid), erucic acid, ricinoleic acid, linoleic acid,linolenic acid, licanic acid, eleostearic acid (octadec-9,11,13-trienoicacid), octadecatetraenoic acid, and octadecatraenoic acid;

Combinations of these fatty acids, especially mixtures found in nature,such a linseed oil fatty acid or tall oil fatty acid;

Sebacates, for example: sebacic acid 1,2-propanediol polyester,di-2-ethylhexy sebacate, dibutyl sebacate, and dioctyl sebacate; and

Tall oil esters, for example: hexyl tallate, 2-ethylhexyl tallate,isooctyl tallate, and octyl epoxy tallate;

The plasticiser or wiping aid is preferably incorporated into the ink ata level of from 0.5% to 10%, more preferably from 3 to 5%, by weight ofthe finished ink.

In order to function properly as an intaglio printing ink, the viscosityof the ink should preferably be controlled within certain limits.Specifically, we prefer that the viscosity of the inks measured at 26°C. and a shear rate of 100 sec⁻¹ should be in the range 20-200 Pascalseconds, more preferably 50-125 Pascal seconds.

The invention also provides a method of producing a document, whichcomprises intaglio printing on a substrate which does not fluoresce inat least the visible region under ultraviolet light using an intaglioprinting ink which includes a photoinitiator comprising an acylphosphineoxide, and curing the ink by exposure to a source of radiant energy.

The substrate is preferably a paper.

The method of the present invention is particularly suitable for theprinting of security documents, such as banknotes. In this case, thepaper or other substrate will be chosen from those materials commonlyknown for use as such documents.

The inks of the present invention can be used on standard intagliopresses fitted with UV lamps, and with a plate temperature of around 40°C.

The invention is further illustrated by the following non-limitingExamples.

EXAMPLES 1 & 2 & COMPARATIVE EXAMPLES 1-

The ingredients shown in the following Table 1 were weighed and mixed toform a paste. The paste was then fully mixed and dispersed using a threeroll mill to produce a homogeneous, viscous paste ink, Mixture A.

TABLE 1 Amount Material (wt %) Supplier Ebecryl 648 (epoxy acrylate)51.3 UCB chemicals Tripropylene glycol diacrylate 1.1 Cray Valley(TPGDA) Chromophtal LGLD (blue 6.8 Ciba pigment) Precipitated CaCO₃ 33.4Omya Corn Starch 2.5 Cerestar Carnauba wax 3.7 Eggar Florstab UV-1 (UVstabiliser) 1.2 Kromachem Total 100

Photoinitiator solutions were made by dissolving the testphotoinitiators into a 50:50 solution of Ebecryl 648 and TPGDA at 33%concentration. These solutions were then mixed with mixture A byblending with a knife to produce the final ink for testing. The inkswere printed using an engraved intaglio plate, using a Harry Rochatproofing press and analysed for the amount of cure and fluorescence. Thefluorescence of the ink for practical purposes at an excitation of 364nm and 254 nm must be less than that emitted by the paper. Fluorescencecan be assessed visually under a light emitting at these wavelengths orby using UV spectrophotometer such as a Perkin Elmer LS50. The inks werecured using a 300 W/inch Ga doped, medium pressure mercury lamp, fromFusion UV.

The cure was assessed by contacting a piece of paper onto cured printusing a hydraulic ram at 10 T spread over a disk with a diameter of 3cm.

The photoinitiators evaluated are listed in Table 2, which also showsfinal ink formulations and the results of the assessment.

TABLE 2 Rahn RCX- Fluorescence Fluorescence Mixture 50:50 02-766 amine(excitation (excitation Example Photoinitiator A PI % Ebecryl 648:TPGDAsynergist Supplier 254 nm) (365 nm) Cure* Comp 1 Esacure ITX 81.3 3.312.9 2.5 Lamberti Yes (bright Yes (bright −1 blue) blue) 1 LucerinTPO^(†) 81.3 3.3 15.4 — BASF No No +1 2 Irgacure 819^(†) 81.3 3.3 15.4 —CIBA No No +2 Comp 2 Benzophenone 81.3 3.3 12.9 2.5 IGM Yes (Dull green)Yes (dull green) −2 Resins Comp 3 Irgacure 369 81.3 3.3 15.4 — Ciba YesYes −2 Comp 4 Diethylthioxanthone 81.3 3.3 12.9 2.5 Lambson Yes Yes −2Comp 5 Photoinitiator 81.3 6.3 12.4 — — Yes Yes +1 combination 1 Comp 6Photoinitiator 81.3 5.4 13.3 — — Yes Yes +2 combination 2 Comp 7 OmnipolTX 81.3 3.3 12.9 2.5 IGM Yes Yes −1 Resins *−2 very poor to +2 very good^(†)acylphosphine oxides according the present invention. ITX is2-isopropylthioxanthone Irgacure 369 is 2 benzyl-2-dimethyl amino-4′morpholinobutyrophenone Irgacure 819 is Bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide Lucerin TPO is Diphenyl(2,4,6-trimethylbenzoyl)phosphine oxide Omnipol TX isPoly(oxy-1,4-butandiyl),α-[[(9-oxo-9H-thioxanthenyl)oxy]acetyl]-ω-[[[(9-oxo-9H-thioxanthenyl)oxy]acetyl]oxy]Photoinitiator Combination 1 Photoinitiator % Esacure ITX 50.00 LucerinTPO 15.00 Irgacure 369 35.00 Photoinitiator Combination 2 Photoinitiator% Esacure ITX 55.6 Irgacure 819 44.4

EXAMPLE 3 Waterwipe Ink

The first three ingredients shown below in Table 3 were mixed togetherusing a Silverson high speed stirrer for approximately 30 minutes untila clear amber varnish was produced. The other ingredients were thenadded to this mixture and mixed to form a paste. The paste was thenfully mixed and dispersed using a three roll mill to produce ahomogeneous paste waterwipe ink.

TABLE 3 Amount Material (wt %) Supplier Ebecryl 657 30 UCB chemicalsSMA1440F 10 Cray Valley Sartomer SR494 17 Cray Valley Microtalc 20.1Omya Irgalite Blue LGLD 5 Ciba Geigy Irgacure 819 4.9 Ciba GeigyCarnauba wax 3 Eggar Florstab UV1 1 Kromachem † Tall oil fatty acid 9VariousThe viscosity data is shown in the following Table 4.

TABLE 4 Shear rate 1/s 2 100 Viscosity Pas at 26° C. 170.1 50.9

EXAMPLE 4 Waterwipe Ink

The first three ingredients shown below in Table 5 were mixed togetherusing a Silverson high speed stirrer for approximately 30 minutes untila clear amber varnish was produced. The other ingredients were thenadded to this mixture and mixed to form a paste. The paste was thenfully mixed and dispersed using a three roll mill to produce ahomogeneous paste waterwipe ink.

TABLE 5 Material Amount (wt %) Supplier Ebecryl 657 30 UCB chemicalsSMA1440F 10 Cray Valley Sartomer SR494 17 Cray Valley Microtalc 20.1Omya Irgalite Blue LGLD 5 Ciba Geigy Lucirin TPO 4.9 Ciba Geigy Carnaubawax 3 Eggar Florstab UV1 1 Kromachem † Tall oil fatty acid 9 VariousThe viscosity data is shown in the following Table 6.

TABLE 6 Shear rate 1/s 2 100 Viscosity Pas at 26° C. 190.4 52.4

EXAMPLE 5 Paperwipe Ink

All the ingredients shown below in Table 7 were added together and mixedto form a paste. The paste was then fully mixed and dispersed using athree roll mill to produce a homogeneous paste paperwipe ink.

TABLE 7 CN104 38.5 Cray valley Sartomer SR494 19.5 Cray Valley Microtalc22.6 Omya Irgalite Blue LGLD 5.5 Ciba Geigy Lucirin TPO 2.4 Ciba GeigyIrgacure 819 2.5 Ciba Geigy Carnauba wax 3 Shamrock Florstab UV1 1Kromachem † Tall oil fatty acid 5 VariousThe viscosity data is shown in the following Table 8.

TABLE 8 Shear rate 1/s 2 100 Viscosity Pas at 26° C. 653.1 54.5

1. An energy curable intaglio security document printing ink, curing byfree radical, acrylate chemistry, and including a photoinitiatorcomprising an acylphosphine oxide, whereby the security documentprinting ink does not fluoresce in at least the visible light wavelengthregion when exposed to ultraviolet light.
 2. A printing ink according toclaim 1, in which said acylphosphine oxide is a compound of formula (I):

in which: R¹ and R² are independently selected from C₁-C₁₂ alkyl groups,C₃-C₇ cycloalkyl groups, aryl groups, aralkyl groups, heterocyclicgroups having from 3 to 7 ring atoms, of which at least one is a sulphuror nitrogen atom and groups of formula —COR³, or R² represents a groupof formula —OR⁴, where R⁴ represents a C₁-C₆ alkyl group, an aryl group,an aralkyl group or a cationic group or atom, or R² represents a groupof formula (II):

where X represents a C₁-C₁₈ alkylene group or a biphenyldiyl group, andR⁵ represents any of the groups represented by R¹ or a group of formula—OR⁴, and R³ represents a C₁-C₆ alkyl group, an aryl group, aheterocyclic group having from 3 to 7 ring atoms, of which at least oneis a sulphur or nitrogen atom, or a group of formula (IV):

where Y represents a C₁-C₁₈ alkylene group a phenylene group, acyclohexylene group or a biphenyldiyl group.
 3. A printing ink accordingto claim 2, in which said acylphosphine oxide is a compound of formula(V):

in which: R¹ represents a C₁-C₁₂ alkyl group, a cyclohexyl group or anaryl group; and R³ is as defined in claim
 2. 4. A printing ink accordingto claim 3, in which each R³ is independently selected from phenylgroups and phenyl groups having from 1 to 4 halogen and/or C₁-C₆ alkyland/or C₁-C₆ alkoxy substituents.
 5. A printing ink according to claim4, in which R¹ represents a C₁-C₁₂ alkyl group or a phenyl group whichis unsubstituted or has from 1 to 3 C₁-C₆ alkyl or alkoxy substituents.6. A printing ink according to claim 2, in which said acylphosphineoxide is a compound of formula (VI):

in which: R¹ and R³ are as defined in claim 2; and R^(2a) represents aC₁-C₁₂ alkyl group, a C₃-C₇ cycloalkyl group, an aryl group, an aralkylgroup, a heterocyclic group having from 3 to 7 ring atoms, of which atleast one is a sulphur or nitrogen atom, or a group of formula —OR⁴,where R⁴ is defined in claim
 2. 7. A printing ink according to claim 2,in which said acylphosphine oxide is a compound of formula (VII):

in which: n is 0 or 1; R⁶ represents a C₁-C₁₂ alkyl group, a C₁-C₆alkoxy group, a phenyl group or a phenyl group having from 1 to 4substituents selected from C₁-C₆ alkyl groups, C₁-C₆ alkoxy groups andhalogen atoms; and R⁷, R⁸, R⁹, R¹⁰, R¹¹ and R¹² are the same as ordifferent from each other and each represents a hydrogen atom, a C₁-C₆alkyl group, a C₁-C₆ alkoxy group or a halogen atom.
 8. A printing inkaccording to claim 2, in which said acylphosphine oxide is2,4,6-trimethylbenzoyl diphenylphosphine oxide,bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, ethyl2,4,6-trimethylbenzoyl diphenylphosphinate orbis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide.
 9. Amethod of producing a security document, which comprises printing on asubstrate which does not fluoresce in at least the visible region underultraviolet light using an intaglio security document printing ink,curing by free radical acrylate chemistry, and which security documentprinting ink includes a photoinitiator comprising an acylphosphine oxideand which security document printing ink does not fluoresce in at leastthe visible light wavelength region when exposed to ultraviolet light,and curing the ink by exposure to a source of radiant energy.
 10. Amethod according to claim 9, in which said radiant energy isultraviolet.
 11. A method according to claim 10, in which saidacylphosphine oxide is a compound of formula (I):

in which: R¹ and R² are independently selected from C₁-C₁₂ alkyl groups,C₃-C₇ cycloalkyl groups, aryl groups, aralkyl groups, heterocyclicgroups having from 3 to 7 ring atoms, of which at least one is a sulphuror nitrogen atom and groups of formula —COR³, or R² represents a groupof formula —OR⁴, where R⁴ represents a C₁-C₆ alkyl group, an aryl group,an aralkyl group or a cationic group or atom, or R² represents a groupof formula (II):

where X represents a C₁-C₁₈ alkylene group or a biphenyldiyl group, andR⁵ represents any of the groups represented by R¹ or a group of formula—OR⁴, and R³ represents a C₁-C₆ alkyl group, an aryl group, aheterocyclic group having from 3 to 7 ring atoms, of which at least oneis a sulphur or nitrogen atom, or a group of formula (IV):

where Y represents a C₁-C₁₈ alkylene group a phenylene group, acyclohexylene group or a biphenyldiyl group.
 12. A method according toclaim 11, in which said acylphosphine oxide is a compound of formula(V):

in which: R¹ represents a C₁-C₁₂ alkyl group, a cyclohexyl group or anaryl group; and R³ is as defined in claim
 11. 13. A method according toclaim 12, in which each R³ is independently selected from phenyl groupsand phenyl groups having from 1 to 4 halogen and/or C₁-C₆ alkyl and/orC₁-C₆ alkoxy substituents.
 14. A method according to claim 13, in whichR¹ represents a C₁-C₁₂ alkyl group or a phenyl group which isunsubstituted or has from 1 to 3 C₁-C₆ alkyl or alkoxy substituents. 15.A method according to claim 11, in which said acylphosphine oxide is acompound of formula (VI):

in which: R¹ and R³ are as defined in claim 11; and R^(2a) represents aC₁-C₁₂ alkyl group, a C₃-C₇ cycloalkyl group, an aryl group, an aralkylgroup, a heterocyclic group having from 3 to 7 ring atoms, of which atleast one is a sulphur or nitrogen atom, or a group of formula —OR⁴,where R⁴ is defined in claim
 11. 16. A method according to claim 11, inwhich said acylphosphine oxide is a compound of formula (VII):

in which: n is 0 or 1; R⁶ represents a C₁-C₁₂ alkyl group, a C₁-C₆alkoxy group, a phenyl group or a phenyl group having from 1 to 4substituents selected from C₁-C₆ alkyl groups, C₁-C₆ alkoxy groups andhalogen atoms; and R⁷, R⁸, R⁹, R¹⁰, R¹¹ and R¹² are the same as ordifferent from each other and each represents a hydrogen atom, a C₁-C₆alkyl group, a C₁-C₆ alkoxy group or a halogen atom.
 17. A methodaccording to claim 11, in which said acylphosphine oxide is2,4,6-trimethylbenzoyl diphenylphosphine oxide,bis(2,4,6-trimethylbenzoyl)phenylphosphine oxide, ethyl2,4,6-trimethylbenzoyl diphenylphosphinate orbis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentylphosphine oxide.
 18. Amethod according to claim 9, in which the substrate is a paper.
 19. Amethod according to claim 9, in which the security document is abanknote.
 20. A method according to claim 12, in which R¹ represents aC₁-C₁₂ alkyl group or a phenyl group which is unsubstituted or has from1 to 3 C₁-C₆ alkyl or alkoxy substituents.
 21. A method according toclaim 9, in which said acylphosphine oxide is a compound of formula (I):

in which: R¹ and R² are independently selected from C₁-C₁₂ alkyl groups,C₃-C₇ cycloalkyl groups, aryl groups, aralkyl groups, heterocyclicgroups having from 3 to 7 ring atoms, of which at least one is a sulphuror nitrogen atom and groups of formula —COR³, or R² represents a groupof formula —OR⁴, where R⁴ represents a C₁-C₆ alkyl group, an aryl group,an aralkyl group or a cationic group or atom, or R² represents a groupof formula (II):

where X represents a C₁-C₁₈ alkylene group or a biphenyldiyl group, andR⁵ represents any of the groups represented by R¹ or a group of formula—OR⁴, and R³ represents a C₁-C₆ alkyl group, an aryl group, aheterocyclic group having from 3 to 7 ring atoms, of which at least oneis a sulphur or nitrogen atom, or a group of formula (IV):

where Y represents a C₁-C₁₈ alkylene group a phenylene group, acyclohexylene group or a biphenyldiyl group.
 22. A printing inkaccording to claim 3, in which R¹ represents a C₁-C₁₂ alkyl group or aphenyl group which is unsubstituted or has from 1 to 3 C₁-C₆ alkyl oralkoxy substituents.